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PubMed1993

Organization of response areas in ferret primary auditory cortex.

1. We studied the topographic organization of the response areas obtained from single- and multiunit recordings along the isofrequency planes of the primary auditory cortex in the barbiturate-anesthetized ferret. 2. Using a two-tone stimulus, we determined the excitatory and inhibitory portions of the response areas and then parameterized them in terms of an asymmetry index. The index measures the balance of excitatory and inhibitory influences around the best frequency (BF). 3. The sensitivity of responses to the direction of a frequency-modulated (FM) tone was tested and found to correlate strongly with the asymmetry index of the response areas. Specifically, cells with strong inhibition from frequencies above the BF preferred upward sweeps, and those from frequencies below the BF preferred downward sweeps. 4. Responses to spectrally shaped noise were also consistent with the asymmetry of the response areas. For instance, cells that were strongly inhibited by frequencies higher than the BF responded best to stimuli that contained least spectral energy above the BF, i.e., stimuli with the opposite asymmetry. 5. Columnar organization of the response area types was demonstrated in 66 single units from 16 penetrations. Consistent with this finding, it was also shown that response area asymmetry measured from recordings of a cluster of cells corresponded closely with those measured from its single-unit constituents. Thus, in a local region, most cells exhibited similar response area types and other response features, e.g., FM directional sensitivity. 6. The distribution of the asymmetry index values along the isofrequency planes revealed systematic changes in the symmetry of the response areas. At the center, response areas with narrow and symmetric inhibitory sidebands predominated. These gave way to asymmetric inhibition, with high-frequency inhibition (relative to the BF) becoming more effective caudally and low-frequency inhibition more effective rostrally. These response types tended to cluster along repeated bands that paralleled the tonotopic axis. 7. Response features that correlated with the response area types were also mapped along the isofrequency planes. Thus, in four animals, a map of FM directional sensitivity was shown to be superimposed on the response area map. Similarly, it was demonstrated in six animals that the spectral gradient of the most effective noise stimulus varied systematically along the isofrequency planes. 8. One functional implication of the response area organization is that cortical responses encode the locally averaged gradient of the acoustic spectrum by their differential distribution along the isofrequency planes. This enhances the representation of such features as the symmetry of spectral peaks and edges and the spectral envelope.(ABSTRACT TRUNCATED AT 400 WORDS)

Acoustic Stimulation↗
PubMed1990

Azimuthal sensitivity of neurons in primary auditory cortex of cats. II. Organization along frequency-band strips.

1. The organization of azimuthal sensitivity of units across the dorsoventral extent of primary auditory cortex (AI) was studied in electrode penetrations made along frequency-band strips of AI. Azimuthal sensitivity for each unit was represented by a mean azimuth function (MF) calculated from all azimuth functions obtained to characteristic frequency (CF) stimuli at intensities 20 dB or more greater than threshold. MFs were classified as contrafield, ipsi-field, central-field, omnidirectional, or multipeaked, according to the criteria established in the companion paper (Rajan et al. 1990). 2. The spatial distribution of three types of MFs was not random across frequency-band strips: for contra-field, ipsi-field, and central-field MFs there was a significant tendency for clustering of functions of the same type in sequentially encountered units. Occasionally, repeated clusters of a particular MF type could be found along a frequency-band strip. In contrast, the spatial distribution of omnidirectional MFs along frequency-band strips appeared to be random. 3. Apart from the clustering of MF types, there were also regions along a frequency-band strip in which there were rapid changes in the type of MF encountered in units isolated over short distances. Most often such changes took the form of irregular, rapid juxtapositions of MF types. Less frequently such changes appeared to show more systematic changes from one type of MF to another type. In contrast to these changes in azimuthal sensitivity seen in electrode penetrations oblique to the cortical surface, much less change in azimuthal sensitivity was seen in the form of azimuthal sensitivity displayed by successively isolated units in penetrations made normal to the cortical surface. 4. To determine whether some significant feature or features of azimuthal sensitivity shifted in a more continuous and/or systematic manner along frequency-band strips, azimuthal sensitivity was quantified in terms of the peak-response azimuth (PRA) of the MFs of successive units and of the azimuthal range over which the peaks occurred in the individual azimuth functions contributing to each MF (the peak-response range). In different experiments shifts in these measures of the peaks in successively isolated units along a frequency-band strip were found generally to fall into one of four categories: 1) shifts across the entire frontal hemifield; 2) clustering in the contralateral quadrant; 3) clustering in the ipsilateral quadrant; and 4) clustering about the midline. In two cases more than one of these four patterns were found along a frequency-band strip.(ABSTRACT TRUNCATED AT 400 WORDS)

Acoustic Stimulation↗
PubMed1991

[Ultrastructural characteristics of callosal neurons in deep layers of the primary auditory cortex (AI) in cat].

An electron microscope study of retrogradely labelled nonpyramidal neurons has been carried out in layers V-VI of the primary auditory cortex (AI) after HRP injections into the contralateral AI of cats. From 2 to 9 synapses were usually revealed on somatic profiles of these callosal neurons. Synapses occupied 15.8 +/- 1.7% (on the average) of the somatic surface of these neurons. All of the revealed synapses on the somata of these callosal neurons had symmetric contacts and were formed by axon terminals with small elongated synaptic vesicles. An average length of these synaptic contacts in sections was 1.6 +/- 0.1 mm. HRP-labelled axon terminals of callosal fibres in layers V-VI contained round synaptic vesicles and formed asymmetric synapses on spines and dendrites. Possible functional significance of axo-somatic synapses in formation of impulsation patterns of the callosal neurons is discussed.

Animals↗
PubMed2001

Probabilistic mapping and volume measurement of human primary auditory cortex.

Despite their potential utility in clinical and research settings, the range of intra- and interindividual variations in size and location of cytoarchitectonically defined human primary auditory cortex (PAC) is largely unknown. This study demonstrates that gyral patterns and the size and location of PAC vary independently to a considerable degree. Thus, the cytoarchitectonic borders of PAC cannot be reliably inferred from macroscopic-MR visible-anatomy. Given the remarkable topographical variability of architectonic areal borders, standard brain mapping which is made solely on the basis of macroanatomic landmarks may lead to structural-functional mismatch. Consequently, interpretations of individual auditory activity patterns might often be inaccurate. In view of the anatomic discrepancies, we generated probability maps of PAC in which the degree of intersubject overlap in each stereotaxic position was quantified. These maps show that the location of PAC in Talairach space differs considerably between hemispheres and individuals. In contrast to earlier cytoarchitectonic work which is based in most cases on studies of single brains, our systematic approach provides extensive microanatomic data as a reference system for studies of human auditory function.

Adult↗
PubMed1987

Distribution of choline acetyltransferase-, serotonin-, dopamine-beta-hydroxylase-, tyrosine hydroxylase-immunoreactive fibers in monkey primary auditory cortex.

Immunohistochemical methods were used to visualize choline acetyltransferase (ChAT)-, serotonin-, dopamine-beta-hydroxylase (DBH)-, and tyrosine hydroxylase (TH)-containing fibers in the primary auditory cortex of the cynomolgus monkey (Macaca fascicularis). Each antiserum revealed a subpopulation of axons with a distinct density and laminar distribution. ChAT-immunoreactive fibers were very dense in superficial layers, particularly in layers I, deep III, and IV, and very sparse in layers V and VI. No immunoreactive cell bodies were evident. Serotonin-immunoreactive fibers were very dense in all cortical layers but exhibited some subtle laminar differences in fiber size and orientation. The densities of DBH- and TH-immunoreactive fibers were substantially lower than the densities of both ChAT- and serotonin-immunoreactive fibers, particularly in layer IV. However, there were substantial differences between the distribution of TH-immunoreactive fibers and that of DBH-immunoreactive fibers. For example, the density of TH-immunoreactive fibers was substantially greater than that of DBH-immunoreactive fibers in layer I. In addition, TH-immunoreactive fibers differed from the other three systems in that TH-immunoreactive fibers exhibited a rostral to caudal gradient of decreasing density. This is the first characterization of the innervation of a specific cortical region by all four of these systems and the first detailed description of the cholinergic innervation of a primate neocortical region utilizing a specific anti-ChAT antiserum. These striking differences in density and laminar distribution suggest that the subcortical extrathalamic systems furnishing these axons differ significantly in their modulation of cortical auditory processing. These data extend observations of previous studies which revealed that the expansion and specialization of the primate neocortex is accompanied by a pronounced regional and laminar differentiation in the intracortical distribution of these highly divergent, extrathalamic afferents.

Adrenergic Fibers↗
PubMed2001

Polysynaptic excitatory pathways induce heterosynaptic depression in the rat auditory cortex.

Short-term plasticity, the effect of a preceding synaptic response on the following response in a pair, in layers II/III of the rat auditory cortex slice after application of repetitive stimuli to layer IV, was investigated using a multichannel extracellular recording system. Paired-pulse depression, which is induced to a moderate degree in standard artificial cerebrospinal fluid, was markedly facilitated in the presence of bicuculline, a GABA(A) receptor antagonist, concurrent with the emergence of a polysynaptic component of the EPSP (polyEPSP) in the first response in a pair. This depression (bicuculline-facilitated synaptic depression, BFSD) was maximal at the minimum interval tested (50 ms), reduced as the interval was increased, and persisted beyond an interval of 2 s. The occurrence of BFSD was dependent on the presence of a polyEPSP regardless of the presence of the monosynaptic component of the EPSP, indicating that BFSD is induced by a heterosynaptic mechanism. D-AP5, an NMDA receptor antagonist, partially eliminated polyEPSPs and reversed BFSD. These results suggest that activation of polysynaptic excitatory pathways induces a heterosynaptic depression in the range of a few seconds and that NMDA receptor activity is involved in this heterosynaptic depression.

2-Amino-5-phosphonovalerate↗
PubMed1995

Multi-electrode array for measuring evoked potentials from surface of ferret primary auditory cortex.

Using silicon-integrated circuit technology, we have fabricated a flexible multi-electrode array and used it for measuring evoked potentials at the surface of the ferret primary auditory cortex (AI). Traditionally, maps of cortical activity are recorded from numerous sequential penetrations with a single electrode. A common problem with this approach is that the state of the cortex (defined in part by level of anesthesia and number of active cells) changes during the time required to generate these maps. The multi-electrode array reduces this problem by allowing the recording of 24 locations simultaneously. The specific array described in this report is designed to record cortical activity over a 1 mm2 area. It is comprised of 24 gold electrodes (40 x 40 microns2) each spaced 210 microns apart. These electrodes are connected to contact pads via gold leads (5 cm in length). The electrodes, leads, and contact pads are sandwiched between two layers of polyimide. The polyimide passivates the device and makes the device flexible enough to conform to the shape of the cortex. The fabrication procedures described here allow various other layouts and areas to be readily implemented. Measurements of the electrical properties of the electrodes, together with details of the multichannel amplification, acquisition, and display of the data are also discussed. Finally, results of AI mapping experiments with these arrays are illustrated.

Acoustic Stimulation↗
PubMed1979

Detection versus discrimination of brief tones by cats with auditory cortex lesions.

In recent years, a number of investigators have provided evidence that the auditory cortex has a critical role in both the detection and discrimination of brief sounds. Dogs and humans with lesions of the neocortical auditory centers have been reported to exhibit significantly elevated detection thresholds for signals shorter than 16 ms in duration. In tests of frequency discrimination, the same subjects also exhibited severe deficits whenever tonal signals were less than 20--40 mn in lengths. In the present report, we present evidence brief tones. Operated cats, while exhibiting normal difference limens for 1-kHz tones of 100-ms duration, have significantly elevated limens for discriminating tones of 8- and 2-ms duration. With further testing, the same operated cats can be shown to have normal absolute thresholds for detecting brief tones.

Animals↗
PubMed1992

Spatial representation of frequency-modulated signals in the tonotopically organized auditory cortex analogue of the chick.

For auditory communication, many birds, including domestic chicks, use a variety of frequency-modulated (FM) sounds. As a first approach to the spatial representation of such sounds in the central auditory system, we have analyzed 2-deoxyglucose (2DG) patterns that were produced by FM stimuli in the tonotopic map of the auditory forebrain area (field L/hyperstriatum ventrale complex) of domestic chicks. Linear FM signals, varying in the depth and range of modulation, and in the direction and rate of the frequency change, were tested. Also included were signals designed to mimic species-specific FM calls. All FM stimuli activated those regions of the map in which frequencies contained in the stimulus spectra were tonotopically represented. However, frequency and amplitude of the FM spectra were not faithfully reproduced by activation of the complete corresponding tonotopic space. FM signals that differed only in the direction of modulation, and therefore had identical long-term spectra, induced maximum 2DG activation at different locations of the tonotopic gradient. FM signals that differed in the rate of change of frequency produced maxima of 2DG uptake at different positions along an isofrequency dimension of the map. These results suggest that the direction of modulation may be represented in a complex fashion along the tonotopic axis of the structure, whereas the rate of change of frequency may be represented along an isofrequency dimension. None of the experiments provided evidence of FM-selective regions within the auditory forebrain complex. However, numerous telencephalic areas, in addition to the primary auditory area, were strongly activated in chicks stimulated with artificial "species-specific" FM signals. These areas could be involved in the processing of biologically relevant stimuli, requiring attention, recognition, and interpretation of the signals.

Acoustic Stimulation↗
PubMedDate not supplied

Neuron discharges in the rat auditory cortex during electrical intracortical stimulation.

Studies were carried out in rats anesthetized with ketamine or nembutal, with recording of multicellular activity (with separate identification of responses from individual neurons) in the primary auditory cortex before and after electrical intracortical microstimulation. These experiments showed that about half of the set of neurons studied produced responses to short tonal bursts, these responses having two components-initial discharges arising in response to the sound, and afterdischarge occurring after pauses of 50-100 msec. Afterdischarges lasted at least several seconds, and were generally characterized by a rhythmic structure (with a frequency of 8-12 Hz). After electrical microstimulation, the level of spike activity increased, especially in afterdischarges, and this increase could last up to 4 h. Combined peristimulus histograms, cross-correlations, and gravitational analyses were used to demonstrate interactions of neurons, which increased after electrical stimulation and were especially pronounced in the response afterdischarges.

Acoustic Stimulation↗
PubMed1994

Evoked potentials recorded from the auditory cortex in man: evaluation and topography of the middle latency components.

The goal of this study is to determine and localize the generators of different components of middle latency auditory evoked potentials (MLAEPs) through intracerebral recording in auditory cortex in man (Heschl's gyrus and planum temporale). The present results show that the generators of components at 30, 50, 60 and 75 msec latency are distributed medio-laterally along Heschl's gyrus. The 30 msec component is generated in the dorso-postero-medial part of Heschl's gyrus (primary area) and the 50 msec component is generated laterally in the primary area. The generators of the later components (60-75 msec) are localized in the lateral part of Heschl's gyrus that forms the secondary areas. The localization of N100 generators is discussed.

Acoustic Stimulation↗
PubMed2005

Principles governing auditory cortex connections.

Topographic maps are common constituents of the primary auditory, visual, and somatic sensory cortex. However, in most cortical areas, no such maps have yet been identified, posing a conceptual problem for theories of cortical function centered on topography. What principle guides the organization of these other areas? We investigated this issue in cat auditory cortex. The connectional topography of five tonotopic areas and eight non-tonotopic areas was assessed using retrograde tract tracing and quantified by three metrics: clustering, dispersion, and separation. Clustering measures the spatial density of labeled neurons, dispersion provides an index of their spread, and separation serves as a scaling metric. These parameters each show that all auditory cortical regions receive precise and equally topographic connections from thalamic, corticocortical, and commissural sources. This isotropic principle suggests a common substrate for coordinating communication across the cortex and may reflect common mechanisms related to the developmental patterning of connections. This unifying principle extends to auditory and prefrontal cortex, and perhaps to other neocortical areas.

Animals↗
PubMed1991

Coding of interaural time differences of transients in auditory cortex of Rattus norvegicus: implications for the evolution of mammalian sound localization.

We obtained quantitative evidence on the coding of interaural time differences (ITDs) of click stimuli by 40 single neurons in the auditory cortex of anesthetized albino rats. Most of the neurons (31/40) received an excitatory input from the contralateral ear, and an inhibitory input from the ipsilateral ear (EI cells). These neurons expressed their sensitivity to ITDs in a sigmoidal relation between spike count and ITD, with maximal responses associated with contralateral-leading ITDs. The mean ITD dynamic range was 590 microseconds. The dynamic ranges typically encompassed at least part of the behaviorally-relevant range (about +/- 130 microseconds). Variations in ITD from 130 microseconds favoring one ear to 130 microseconds favoring the other ear caused spike response rate changes, on average, of 29.5%. These data are similar to those previously presented for the central auditory systems of larger mammals, whose auditory localization acuity is significantly better than that of the rat. We argue, therefore, that the sound localization mechanisms based on transient ITDs have not evolved in a fashion that covaries with interaural distance, and that there exists a mismatch between the ITDs the rat will encounter in the free field, and the ITDs which are encoded by its nervous system. This may be one reason why sound localization acuity has a roughly inverse relation to interaural distance.

Acoustic Stimulation↗
PubMed1988

Postnatal development of lamina III/IV nonpyramidal neurons in rabbit auditory cortex: quantitative and spatial analyses of Golgi-impregnated material.

We have studied the postnatal development of lamina III/IV spine-free nonpyramidal neurons in the auditory cortex of the New Zealand white rabbit. The morphology and dendritic branching pattern of single cells impregnated with a Golgi-Cox variant were analyzed with the aid of camera lucida drawings and three-dimensional reconstructions obtained with a computer microscope. Sample sizes of 20 neurons were obtained at birth (day 0), postnatal day (PD) 3, 6, 9, 12, 15, 21, and 30 days of age. Normative data were also available from PD-60 and young adult rabbits studied previously. At birth, lamina II-IV have not yet emerged from the cortical plate; immature nonpyramidal neurons at the bottom of the cortical plate (presumptive layer IV) are characterized by short, vertically oriented dendrites. Growth-cone-like structures are present along the shafts and at the tips of the dendrites. At birth, soma area and total dendritic length are, respectively, 34 and 10% of adult values. The cortical plate acquires a trilaminar appearance at PD-3. The six-layered cortex is present by PD-6. During the first postnatal week dendritic length increases fourfold and is accompanied by a significant increase in both terminal and preterminal dendritic growth cones. At the onset of hearing at PD-6, there is a significant proliferation of dendrites and branches to 144 and 200% of adult levels, respectively. These supernumerary dendrites are rapidly lost during the second postnatal week, at which time the somata and dendrites become covered with spines. The loss of higher-order dendrites occurs more gradually; the number of dendritic branches is still 116% of adult values at PD-30. Spine density peaks between days PD-12 and PD-15, and then gradually diminishes until the cells are sparsely spined or spine free by PD-30. Total dendritic length increases in a linear fashion up to PD-15, at which time it is 80% of adult values. An analysis of terminal and intermediate branches demonstrated that the increase in total dendritic length after PD-6 is due entirely to the growth of terminal dendrites. Total dendritic length attains adult levels by PD-30. Spatial analyses revealed that a vertical orientation of dendrites is present at birth. Associated with the onset of hearing at PD-6, there is an explosive elaboration of dendrites toward the pial surface.(ABSTRACT TRUNCATED AT 400 WORDS)

Aging↗
PubMed1990

Delay-tuned neurons in auditory cortex of mustached bat are not suited for processing directional information.

1. The mustached bat, Pteronotus parnellii parnellii, emits bisonar pulses each consisting of eight components: CF1-4 and FM1-4. In the auditory cortex of the bat there are arrays of FM-FM neurons that are tuned to particular delays of echo FMn (n = 2, 3, or 4) from pulse FM1. They are specialized for the processing of target-range information. The FM signal is suited for ranging and also for target localization. Therefore we studied the directional sensitivity of FM-FM neurons with pulse FM1 and echo FMn. One of the FM1-FMn pair was moved around the bat's head while the other was fixed in front of the bat. 2. FM-FM neurons are sharply tuned in echo delay and are broadly tuned in echo amplitude. That is, they are tuned to a target that has a particular cross-sectional area and that is located at a particular distance from the bat. Their best amplitudes for echoes range between 8 and 73 dB sound pressure level (SPL). The best amplitude is approximately 30 dB higher than minimum threshold in the majority of neurons. 3. The higher the best amplitude is relative to minimum threshold, the larger the receptive field is at the best amplitude. The receptive field of FM-FM neurons at 30 dB above minimum threshold is always so large that it covers the entire contralateral auditory field or the entire contralateral field and the medial half of the ipsilateral auditory field. The large size of the receptive field and the uniform distribution of response magnitudes within the receptive field indicate that FM-FM neurons are not suited for sound localization. Directional information is probably processed in parallel by a separate population of neurons other than FM-FM neurons. 4. The receptive field of FM-FM neurons at 10 dB above minimum threshold is much smaller than that at 30 dB above minimum threshold, but it is still large. The mean azimuthal and elevational widths for echo FMn are greater than 70 degrees in all directions. There is no sign that FM-FM neurons are more directional than peripheral neurons. Furthermore, there is neither an azimuthal nor an elevational axis within the FM-FM area. 5. Mean best azimuths of FM-FM neurons are different for each echo FM harmonic: lateral 35 degrees for FM2 and lateral 19 degrees for FM3 and FM4.(ABSTRACT TRUNCATED AT 400 WORDS)

Acoustic Stimulation↗
PubMed2001

Tetanic stimulation and metabotropic glutamate receptor agonists modify synaptic responses and protein kinase activity in rat auditory cortex.

We investigated whether tetanic-stimulation and activation of metabotropic glutamate receptors (mGluRs) can modify field-synaptic-potentials and protein kinase activity in rat auditory cortex, specifically protein kinase A (PKA) and protein kinase C (PKC). Tetanic stimulation (50 Hz, 1 s) increases PKA and PKC activity only if the CNQX-sensitive field-EPSP (f-EPSP) is also potentiated. If the f-EPSP is unchanged, then PKA and PKC activity remains unchanged. Tetanic stimulation decreases a bicuculline-sensitive field-IPSP (f-IPSP), and this occurs whether the f-EPSP is potentiated or not. Potentiation of the f-EPSP is blocked by antagonists of mGluRs (MCPG) and PKC (calphostin-C, tamoxifen), suggesting that the potentiation of the f-EPSP is dependent on mGluRs and PKC. PKC antagonists block the rise in PKC and PKA activity, which suggests that these may be coupled. In contrast, ACPD (agonist at mGluRs) decreases both the f-EPSP and the f-IPSP, but increases PKC and PKA activity. Quisqualate (group I mGluR agonist), decreases the f-IPSP, and increases PKA activity, suggesting that the increase in PKA activity is a result of activation of group I mGluRs. Additionally, the increase in PKC and PKA activity appears to be independent of the decrease of the f-EPSP and f-IPSP, because PKC antagonists block the increase in PKC and PKA activity levels but do not block ACPD's effect on the f-EPSP or f-IPSP. These data suggest that group I mGluRs are involved in potentiating the f-EPSP by a PKC and possibly PKA dependent mechanism which is separate from the mechanism that decreases the f-EPSP and f-IPSP.

6-Cyano-7-nitroquinoxaline-2,3-dione↗
PubMed2002

Superposition of horseshoe-like periodicity and linear tonotopic maps in auditory cortex of the Mongolian gerbil.

The segregation of an individual sound from a mixture of concurrent sounds, the so-called cocktail-party phenomenon, is a fundamental and largely unexplained capability of the auditory system. Speaker recognition involves grouping of the various spectral (frequency) components of an individual's voice and segregating them from other competing voices. The important parameter for grouping may be the periodicity of sound waves because the spectral components of a given voice have one periodicity, viz. fundamental frequency, as their common denominator. To determine the relationship between the representations of spectral content and periodicity in the primary auditory cortex (AI), we used optical recording of intrinsic signals and electrophysiological mapping in Mongolian gerbils (Meriones unguiculatus). We found that periodicity maps as an almost circular gradient superimposed on the linear tonotopic gradient in the low frequency part of AI. This geometry of the periodicity map may imply competitive signal processing in support of the theory of "winner-takes-all".

Acoustic Stimulation↗